478 THE AMERICAN NATURALIST [Vol. XLVIII 



Finlayson's squirrel seems to have accustomed itself by 

 gradual stages to the white condition, so that it is prob- 

 ably not at a great disadvantage by reason of its 

 whiteness. 



Pigment Patches in Birds 



In birds the same primary pigment patches seem to be 

 present as in mammals, and they are homologous in the 

 two groups. In denning the extent of the pigment patches, 

 however, allowance must be made for the fact that the long 

 feathers may cover a part of the body remote from their 

 origin. The distribution of the feathers or the pterylosis 

 of the species in hand must also be remembered. In 

 order to arrive at the true interpretation of the patches, 

 it is necessary to consider the pigment as projected back 

 from the vanes of the feathers to the part of the body 

 at their bases. By so doing, it becomes evident that a 

 feather variegated with pigmented and unpigmented (or 

 white) areas indicates none the less that the feather 

 arises from a place of pigment formation. It is only a 

 wholly white feather or patch of feathers that can be 

 considered albinistic in the sense here intended. The 

 factor determining the intermittent formation of pig- 

 ment in the individual feather is probably a wholly 

 different one from that determining the presence or ab- 

 sence of pigment formation at certain places on the 

 body, though not necessarily different except in its inter- 

 mittent action. 



In the domestic pigeon of our streets and buildings, 

 we have a species that in its wild state is normally fully 

 pigmented except for a white rump patch. Under semi- 

 domestication it has developed partial albinism to a large 

 degree, so that it is possible to obtain a complete series 

 representing on the one extreme a totally pigmented 

 bird without a trace even of the white rump patch, and 

 on the other extreme a bird of pure white plumage. A 

 few of the intermediate stages in areal reduction of pig- 

 mentation are shown in Figs. 49 to 53, selected from 

 birds raised for the market and, so far as known, not 



